1. Field of the Invention
The present invention relates to the area of steels for application in the field of metal containers for food, non-food products or industrial purposes.
2. Discussion of the Background
The steels smelted for uses specific to metal containers differ from thin sheets in particular by their physical characteristics.
The thicknesses of steel sheets for containers vary from 0.12 mm to 0.25 mm for the great majority of uses, but can reach greater thicknesses, as much as 0.49 mm, for very special applications. This is the case, for example, of certain containers for non-food products, such as certain aerosols, or the case of certain industrial containers. Their thickness can also be as small as 0.08 mm, in the case of food receptacles, for example.
Steel sheets for containers are usually coated with a metal coat (tin which may or may not be remelted, or chrome), on which there is generally deposited an organic coat (varnish, inks, plastic films).
In the case of two-piece containers, these are made by deep-drawing under a blank holder or by deep-drawing/trimming for beverage cans, and are generally cylindrical or frustoconical, axially symmetric cans. The container designers are showing increasing interest in even thinner steels, however, with thickness from 0.12 mm to 0.075 mm and, with the objective of distinguishing themselves from the competitors, they are trying to introduce increasingly more complex shapes. Thus we now find cans of original shapes, manufactured from steel sheets of small thicknesses, which sheets, even though presenting greater forming difficulties, must meet the use criteria (mechanical durability of the containers, resistance to the axial load to which they are subjected during storage in stacks, resistance to the internal overpressure to which they are subjected during sterilizing heat treatment and to the internal partial vacuum to which they are subjected after cooling) and therefore must have very high mechanical strength.
Thus the use and performance of these containers are believed to depend on a certain number of mechanical characteristics of the steel:                coefficient of planar anisotropy, ΔC aniso,        Lankford coefficient,        yield strength Re,        maximum rupture strength Rm,        elongation A %,        distributed elongation Ag %.        
To impart to the container equivalent mechanical strength at smaller steel thickness, it is preferable that the steel sheet present a higher maximum rupture strength.
It is known that containers can be made by using steels with low aluminum content, and in particular steels known as “renitrided low-aluminum steels”. Such a steel is, for example, described in French Patent Application No. 95-11113.
The carbon content usually sought for this type of steel ranges between 0.050% and 0.080%, the manganese content between 0.20% and 0.45%. The aluminum content is controlled to a value of less than 0.020% with the objective of imparting to the steel sheet an improved microstructure, good freedom from inclusions and, consequently, high mechanical characteristics.
The nitrogen content is also controlled, and ranges between 0.008 and 0.016%. This nitrogen content is ensured by addition of calcium cyanamide to the ladle during smelting of the steel, or by blowing gaseous nitrogen into the steel bath. The known benefit of the nitrogen addition is to harden the steel by solid solution effect.
These steel sheets are made by cold rolling a hot strip to a cold-rolling ratio of between 75% and more than 90%, followed by continuous annealing at a temperature of between 640 and 700° C., and a second cold-rolling with a percentage elongation which varies between 2% and 45% during this second cold-rolling depending on the desired level of maximum rupture strength Rm.
For steels with low aluminum contents, however, high mechanical characteristics are associated with poor elongation capacity. This poor ductility, apart from the fact that it is unfavorable to forming of the container, leads during such forming to thinning of the walls, a phenomenon which will be unfavorable to the performances of the container.
Thus for example, a “renitrided low-aluminum” steel with a maximum rupture strength Rm on the order of 550 MPa will have a percentage elongation A % on the order of only 2 to 5%.